The present invention relates to an electric power steering apparatus.
Conventionally, electric power steering (EPS) apparatus using a motor as a drive source have been known as vehicle power steering apparatuses. In a typical EPS apparatus, various compensation control procedures are executed to further improve the steering feel.
For example, the state of a steering operation executed by the driver (steering state) includes “turning” for increasing the steering angle, “holding” for maintaining the steering angle, and the “returning for reducing the steering angle. The steering feel experienced by the driver changes in each steering state. The steering feel tends to change significantly during a transition between steering states.
Accordingly, Japanese Laid-Open Patent Publication No. 2006-142932 discloses an EPS apparatus that executes compensation control procedure according to the steering state. Prior to the computation of a basic assist control amount, the EPS apparatus disclosed in the publication corrects the value of steering torque, which will be used as a base of the computation, in accordance with the steering state (steering torque shift control). That is, by simply correcting the steering torque prior to the computation of the basic assist control amount, assist performance that is appropriate for each steering state is achieved. Specifically, in the “holding” and in the “returning”, the steering torque shift control is executed such that the basic assist control amount (the absolute value thereof) is increased. This reduces the load on the driver required for the “holding”. Also, the driver is prevented from feeling disturbed when the steering state is switched from the “holding” to the “returning”, so that favorable steering feel is maintained.
When executing the compensation control procedure, a compensation component is passed through a low pass filter (LPF), so that sharp changes in assist force is inhibited. Therefore, favorable steering feel is achieved.
However, in cases where the steering torque shift control is executed to increase the assist force during the “holding” and the “returning”, the execution of the filtering process for preventing the assist force from being sharply changing (sharp change prevention) can degrade the steering feel.
That is, in many cases, the steering operation is performed in the order of the “turning” from the neutral position of the steering wheel, the “holding”, and the “returning” to the neutral position. Thus, the sharp change prevention is particularly effective in inhibiting an sharp change of the assist force when the steering state is switched from the “holding” to the “returning”.
However, in reality, the steering state is changed directly to the “turning” from the “holding” or from the “returning” without a sufficient time at the neutral position in some cases. In such a case, the execution of the sharp change prevention process retards the reduction of the assist force, which has been increased during the “holding” or the “returning. As a result, the assist force becomes excessive, and the steering feel experienced by the driver is degraded. Degraded steering feel includes excessively small resistance during “turning,” accompanying reduction of road information, and a reduced sensation of the “turning” subsequent to the “holding” or the “returning”. Thus, the above compensation control still has room for improvement.